1. Field of the Invention
The present invention relates to a conductive particle, an anisotropic concoctive film, a joined structure, and a joining method.
2. Background of the Invention
To connect circuit members each other, such as a connection between a liquid crystal display device and a tape carrier package (TCP), a connection between a flexible printed circuit (FPC) and a printed circuit board, or a connection between a FPC and a printed circuit board, a circuit connecting material (e.g., anisotropic conductive adhesive), in which conductive particles are dispersed in an adhesive, is used. In recent years, when a semiconductor silicon chip is mounted on a substrate, in order to connect circuit members to each, so-called “flip chip mounting” is employed in which the semiconductor silicon chip is directly bonded face down on the substrate without using a wire bond. In this flip chip mounting, circuit connecting materials, such as an anisotropic conductive adhesive, are used for connecting circuit members to each other.
The above-mentioned circuit connecting material typically contain an adhesive composition and conductive particles. Examples of the conductive particles include (1) conductive particles containing, on surfaces of nickel particles, at least one kind selected from Au, Ag and Pd, via an alloy layer (for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-25345); (2) conductive particles which include a core made of an organic polymer, a metal layer which is formed on the core and made of copper, nickel, a nickel alloy, silver or a silver alloy, and an outermost surface layer made of gold or palladium (for example, International Publication No. WO2005/002002); (3) conductive particles which include a core made of an organic polymer, a metal layer which is formed on the core and made of copper, nickel, a nickel alloy, silver or a silver alloy, an outermost surface layer made of gold or palladium, and protruded portions provided on the front surface side of the outermost surface layer, having a height of 50 nm to 500 nm and composed of nickel, copper, gold, cobalt or the like (for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-166438); and (4) conductive particles resin fine particles, and a conductive layer formed on surfaces of the resin fine particles and made of nickel, wherein the conductive layer having, on its surface, protrusions containing at least one metal or a metal oxide selected from the group consisting of gold, silver, copper, palladium, zinc, cobalt and titanium, as a core material (for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-35573). Here, in order to properties of the conductive particles to the maximum extent possible, it is the most preferable to compress conductive particles to 40% to 70% to the original volume prior to being used for connecting. Note that provision of the protrusions on the surfaces of conductive particles is preferred for an inactive metal which is hard and does not have surface oxidation properties to be connected with low resistance.
However, in the case where nickel is used as an outermost shell of conductive particles, there is a problem that the surface of nickel is easily oxidized and thus a low resistance cannot be obtained.
In the case where palladium is used as an outermost shell of conductive particles, when the conductive particles are compressed to an appropriate extent prior to being used, there is a problem that the rise in resistance after environmental reliability test is further increased due to plating cracking.
Further, in the case where cobalt, which is higher in Mohs hardness than palladium, is used as an outermost shell of conductive particles, there are problems that cobalt is highly surface active and easily oxidized, and when used in combination with a radically polymerizable binder, a catalyst action is caused, which leads to a significant degradation in the storage stability.
In addition, recently, IZO (indium zinc oxide) tends to be used in place of ITO (indium tin oxide), which increases production cost.
In the case where gold which has high conductivity is used as an outermost shell of conductive particles, the conductive reliability can be ensured with respect to an ITO substrate, which has been conventionally used as an electrode material, but with respect to an IZO substrate having a metal oxide (nonconductor) formed on its surface and thus having a smooth surface, the gold broke through the metal oxide (nonconductor), it is impossible to make the metal oxide bite into the gold, and thus adequate conductive reliability cannot be ensured.
At the present time, conductive particles which can reduce the stress while maintaining high hardness (hardly causing cracks even in a state of being crushed in connection process) and can ensure adequate conductive reliability not only with respect to ITO substrates, but also with respect to IZO substrates have not yet been found out.